Studies of the development of the mammary gland of 50-day old female rats have established the basic pattern of cell evolution. Studies of permanent lines of rat mammary cells in vitro have revealed a pattern of cell differentiation in vitro similar to that taking place in vivo. Antigenic markers distinguishing two cell types in vitro have been found. Further progress requires additional markers. It is proposed to look for organ-specific, differentiation-specific, and tumor-specific antigens in mammary cells using the monoclonal antibody technique. Preliminary work by the applicant shows that antibodies which recognize mammary cells can be readily obtained by this technique. However, the construction of cell lines producing discriminating antibodies requires a considerable effort because they can be expected to constitute only a small fraction of all antibody-producing clones.